The present invention relates to fiber optics as well as to measuring gain in amplified fiber optic spans.
It is known in the art that one manner of obtaining additional optical gain in an amplified fiber optic span is by pumping the line fiber with high-energy photon (low wavelength) lasers. In the resulting xe2x80x9cRaman effectxe2x80x9d, high-energy photons are absorbed by the fiber and may be re-emitted in the form of stimulated emission in the presence of longer wavelength photons, thus resulting in amplification. The gain profile for the stimulated re-transmission has a finite width. Signal gain across a wide spectral profile may involve the superposition of gain profiles for several Raman pumps.
Using adequate pump wavelengths to provide gain across a signal band, a known model can be derived to predict the relative pump powers required to achieve a desired gain profile through pump gain profile superposition, while taking into account several secondary non-linear fiber optic effects as well.
In the prior art, a closed loop system has been employed which monitors system span profile parameters for gain derivation. The desired Raman gain is then estimated on the basis of a modeled solution. However, this implementation does not take into account the dynamic nature of the system span profiles. Consequently, in cases where the system span profiles are dynamic in nature, the known gain control model (algorithm) will fail. Furthermore, Raman gain estimation on the basis of a modeled solution often itself employs an open loop gain implementation, and the resulting gain profile is not corrected.
U.S. Pat. No. 6,081,323 issued to Mahgerefteh et al. on Jun. 27, 2000, teaches a method and apparatus for the measurement of Raman gain spectrum in an optical fiber. This patent, however, describes the determination of Raman gain spectrum and is silent on the measurement of Raman gain.
Therefore, there is a need for an improved method of calculating Raman gain which permits dynamic adjustments to system parameters, thus correcting gain inaccuracies induced by the application of known models and permitting an accurate derivation of the Raman gain.
Accordingly, it is an object of the invention to provide a closed loop control strategy using system wavelength profile information to provide accurate derivation of Raman gain in a fiber optic span.
It is another object of the invention to provide a manner in which Raman gain may be measured in real-time, in service.
The term xe2x80x9cpower profilexe2x80x9d will be used herein as a generic term to describe either a gain profile or a loss profile.
According to one aspect of the invention, there is provided a method of dynamically determining a Raman gain profile of an optically amplified fiber optic span, said method comprising the steps of: measuring a power profile for each of a plurality of system components; transmitting the measured power profiles to a central location; transmitting changes in the measured power profiles to the central location; and dynamically calculating the Raman gain profile for the system based on the changes in the measured power profiles.
According to another aspect of the invention, there is provided a system for dynamically determining a Raman gain profile of an optically amplified fiber optic span, said system comprising: optical spectrum analyzers for measuring a power profile of said fiber optic span and of a plurality of system components so as to determine the existence of a loss or a gain therein; means for receiving the measured power profiles from the optical spectrum analyzers, and for receiving changes in the measured power profiles; and means for dynamically calculating the Raman gain profile for the system based on the changes in the measured power profiles.